The present invention relates to heat reservoir devices and, in particular, such devices as are used in the food service industry and commonly called "pellets".
Pellets have long been used in the food service industry as a means for maintaining the desired temperature of comestibles on a plate or other container until such time as it is to be consumed. Originally pellets were a "donut" shaped piece of metal which was heated, placed in a stainless steel shell and the plate placed on the top. Both the plate and the pellet were contained by the metal shell. A cover was then placed over the plate. The pellets were previously heated in a heater which would heat the pellets to the desired temperature and allow them to be placed into the stainless steel metal shell as noted. Such pellets would then maintain the temperature of the food in the dish at a desirable heated temperature until it was consumed. Such pellets were commonly used in hospitals, nursing homes, and the like where food is prepared and placed on plates in a central kitchen and then placed in carts to be dispensed to the consumers throughout the particular hospital, nursing home, and the like. This is now a standard means of maintaining the food at a proper temperature until such time as it is placed before the patient or consumer. These metal pellets were not only expensive, but heavy and difficult to handle.
Consequently, improvements were needed, one of which was a unitized base which consisted of two stainless steel shells with an iron disc sandwiched in between. Some of these new style pellets also had insulation between the bottom outer layer and the disc. Further, single piece bases were also used made of either aluminum or stainless steel, some coated and some of the aluminum bases were anodized.
All of the foregoing pellets worked on the principle of heating a mass of metal so as to act as a heat radiator to keep the food hot for a period of time.
These again were unsuitable because of the cost and weight of the metal, and their inefficient radiation properties.
Another style of pellet that came on the market was a wax-filled pellet which consisted of two steel shells welded together with wax, similar to paraffin wax, sealed inside. These pellets worked on the principle of phase change. The wax, when heated, absorbs the heat energy as it is transformed from a solid to a liquid and as it slowly goes back from a liquid to a solid it gives up that heat energy, which energy is absorbed by a plate and food therein sitting in the pellet. A variation of this type of pellet was one made of two plastic shells sealed together with dead air space therebetween.
Again, the cost of making such plates was excessive, and the amount of wax that could be included was limited so again there was not the desired degree of heat transfer available.
In an attempt to cut costs by making products which were easily manufactured and with less weight, efforts were made to use plastic pellets made of plastics having a slow rate of heat PG,4 transfer. An example of such is melamine which has a slow degree of heat transfer. Once heated it gives up its heat very slowly so when the heated plate with food and its cover is placed on the pellet it helps to maintain the food temperature at the desired temperature for a longer period of time. However, melamine pellets are undesirable in that when repeatedly heated in the devices commonly used to heat pellets they tend to shrink and become brittle. As a consequence, special heating devices are required which add steam to the heat source as a means of preventing the shrinking and drying of the melamine. Such special heating device adds to the cost.